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Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-versi

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max"
Date: Thu, 12 Apr 2018 09:19:54 +0100
User-agent: Mutt/1.9.2 (2017-12-15)

On Wed, Apr 11, 2018 at 05:35:55PM +0200, Andrea Bolognani wrote:
> On Tue, 2018-04-10 at 09:52 +0100, Daniel P. Berrangé wrote:
> > On Tue, Apr 10, 2018 at 09:41:33AM +0200, Andrea Bolognani wrote:
> > > I figure the people not explicitly specifying a CPU model on the
> > > command line will probably also use '-M virt' instead of versioned
> > > machine types, which means they will get a different guest behavior
> > > after upgrading QEMU regardless.
> > 
> > Libvirt uses versioned machine types and does not specify -cpu unless the
> > user has added <cpu> to their XML. IOW libvirt assumes the default CPU
> > model is stable because that's what QEMU has promised in the past.
> Hm, you have a point.
> I wonder how well that works in practice, though. I started a guest
> with no <cpu> element on my laptop and it ended up having
>   vendor_id    : GenuineIntel
>   cpu family : 6
>   model      : 6
>   model name : QEMU Virtual CPU version 2.5+
>   stepping   : 3
> which I guess translates to the qemu64 CPU model, based on the
> description. I have verified the -cpu option is not present on the
> command line.
> The name seems to imply that if I were using a QEMU release older
> than 2.5 I would get a different CPU model, but maybe the stable CPU
> guarantee you mention is just a fairly recent development.

That model ID string is tied to the machine type, so it is stable. Older
machine types use PC_CPU_MODEL_IDS() in pc.h to ensure they have not

> I also know that ppc64 performs some trickery if you don't specify a
> CPU model, so by default you get a behavior which is pretty close to
> using -cpu host.
> Basically I'm wondering how reasonable it is to expect a migratable
> machine and a stable guest ABI when relying on QEMU defaults instead
> of explicitly picking a CPU model.

Provided you have picked a versioned machine type, it is expected that all
other defaults are stable, and that includes CPU models.

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